1. Field of the Invention
Telescoping crane booms and particularly those mounted on mobile carriers require the efficient use of a minimum of material, i.e. structural steel, to resist the deflection and stresses imposed on the boom by lifting the required loads. The box section or rectangular cross-section boom, which is universally used, is an efficient structure to resist the bending loads, the major strength in the box section being obtained from the material or thickness in the top and bottom sections. Telescoping cranes consist of a number of similar sections nested together, usually between two and five sections, and a smaller section is extensible and retractible within the next larger section. When telescoped in or out, the boom sections support each other with sliding pads, shoes or rollers interposed between the sections. The maximum deflection and stress in the boom sections and the maximum load imposed on the bearing points between sections occur when the sections are telescoped out their maximum distance. The top and bottom members of the box section boom construction are highly stressed, the top in tension and the bottom in compression, due to bending from the overhanging load, and, in addition, these members are subject to high local loading at the bearing supports. The present invention contemplates a more efficient boom section for carrying higher loads by locating the bearing points as close as possible to the neutral axis of the boom. In doing so, the load bearing supports will be located where the bending stresses are at or near a minimum, and the top and bottom members of the boom sections will be relieved of the local high bearing loading and hence can be sized to carry the tension and compression stresses, respectively, due to bending.
2. Description of the Prior Art
In order to provide for free guided relative longitudinal movement between adjacent sections of the boom structure, normally a pair of bearing surfaces are provided on the first inner section, on its top side adjacent the rear end thereof for contact with under side of the top wall of the largest or base section. Likewise, a pair of bearing surfaces are normally provided on the forward or outer end of the base section for contact with the underside of the bottom wall of the first inner section. This alternating positioning of bearing surfaces is used for additional boom sections as will be more fully explained in connection with the present invention. An example of this arrangement using stationary and pivotally mounted bearing pads is shown in Johnston et al. U.S. Pat. No. 3,368,696. The use of rollers as bearings between sections is typically shown in Obenchain U.S. Pat. No. 2,819,803 and Grove U.S. Pat. No. 3,386,594. An articulated bearing assembly utilizing a pair of pivotally mounted slider blocks or shoes is shown in Benkowski U.S. Pat. No. 3,782,790. Examples of modifying the rectangular or box-shaped cross-section to obtain greater load carrying capacity are shown in Sterner U.S. Pat. No. 3,708,937 and Johnston U.S. Pat. No. 3,807,108 which utilize trapezoidal cross-sections. A beveled box section is shown in Eiler U.S. Pat. No. 3,481,490. There is no suggestion in any of these prior art structures of locating the bearing surfaces at or close to the neutral axis of the boom cross-section.